Immune system response to infection varies

Q: Once you get a disease such as chickenpox or the flu, how does it change your immune system?

A: Your immune system is like an army that is ready to spot and stop invaders like the viruses that cause chickenpox and the flu. Your immune system also goes to war when an old infection comes back to life. But after the battle is over, your immune system quiets down and waits for the next fight.

When a virus like those that cause chickenpox and the flu enters your body, specialized cells in your immune system recognize it as foreign, and start to attack. Some immune system cells make antibodies. These molecules float in the blood and attach to any viruses that are in the blood to destroy them.

Other immune system cells recognize that the virus has entered some of your cells because pieces of the virus stick out of the cell. Your immune system cells attack and kill your infected cells because the virus can only multiply and thrive inside your cells. Your immune system stops the virus from multiplying by killing the cells where it lives. You have plenty of uninfected cells, so killing a few infected ones usually doesn't hurt you.

Usually, the parts of your body that may have been injured by the virus infection heal and recover fully. That is true of the flu, for example.

The response of the immune system varies depending upon the specific infectious organism and whether the immune system is healthy vs. compromised.

Sometimes a virus causes lasting damage, even in people with healthy immune systems. Polio, a disease caused by the poliovirus, can cause paralysis before the virus is eliminated from the body. Fortunately, because of immunizations, polio is rare in most developed nations.

Sometimes the immune system cannot eliminate a virus. Varicella- zoster virus, the cause of chickenpox, is an example. You won't get chickenpox again. However, once you are infected, the virus remains in your body for the rest of your life. The immune system usually keeps it from becoming active and causing trouble.

But this protection is not perfect. After causing chickenpox, the virus settles down in some of our nerve cells. It can become active many years later to cause the rash and pain of the disease commonly called "shingles." When the virus reactivates, the immune system typically attacks it again and beats it back into a quiet state.

Influenza is a special problem because the virus that causes the flu has the ability to change its genetic makeup easily, and it does so frequently. So having the flu in the past caused by one strain, won't protect you in the future.

(Anthony L. Komaroff, M.D., is professor of medicine and editor-in-chief of Harvard Health Publications at Harvard Medical School. Dr. Komaroff has served on various advisory committees to the federal government, and is an elected Fellow of the American Association for the Advancement of Science.)